(1) Field of the Invention
This invention relates to the electric range art and particularly to the use of foil heaters with a solid utensil-supporting cover plate.
(2) Description of the Prior Art
Solid plate surface heating units have been provided in the past with glass-ceramic plates having film heaters of noble metal bonded directly to the underside of the plates. Three examples of glass-ceramic heating units using film heaters are shown in the Hurko U.S. Pat. Nos. 3,067,315 and 3,883,719, both assigned to the present assignee, and in the Brouneus U.S. Pat. No. 3,813,520. Such film heaters are of serpentine shape, and they are bonded directly to the plate. They provide a most efficient heating system for glass-ceramic surface heating units or cooktops because the film strips have a very low thermal mass and good thermal coupling with the plate, resulting in quicker response to heat-up and cool-down conditions. The film heater stores very little heat, and it radiates very little heat in a downward direction because of its low emissivity surface. One disadvantage of the film heater designs for solid plate surface heating units is the relatively high cost of film materials because they are of noble metals, such as gold and platinum.
Etched foil heaters for use with solid plate surface heating at low temperatures below about 450.degree. F. have been available before this invention from Safeway Products, Inc. of Middletown, Conn. They are highly reliable at relatively low temperatures, and their cost is a great deal below film heaters. An example of etched foil heaters is given in the Howie U.S. Pat. No. 3,869,596, which is assigned to Safeway Products, Inc. This patent shows a glass-ceramic plate with an etched foil heater bonded between two layers of dielectric material, and this heating element is bonded directly to the underside of the glass-ceramic plate, and, as stated in this Howie patent, it has an anticipated operating temperature on the order of 450.degree. F.
In experimenting with high-temperature applications for etched foil heaters, we made improvements by eliminating the bonding of the laminated foil heating directly to the glass-ceramic plate. The bonding action was a deterrent because it limited the kind of foil material used to a low thermal expansion metal foil so as to be able to match the coefficient of thermal expansion of the glass-ceramic plate, and this in turn limited the application of foil heaters to a low temperature range having a maximum of about 450.degree. F.
Moreover, prior art foil heaters used a flexible, thermosetting, organic adhesive to make a strong bond, which again limited the foil heater to a low temperature range. At higher temperatures, such adhesive would carbonize and cause short circuits between the turns of the foil heater as was experienced during the tests.
The principal object of the present invention is to provide a solid plate surface heating unit with a separate, flexible, insulated foil heater for use at high temperatures in the vicinity of 1250.degree. F.
A further object of the present invention is to provide a solid plate surface heating unit of the class described with means to allow the resistive foil heater to freely expand at operating temperatures with relation to its supporting laminations of insulation.
A further object of the present invention is to provide a solid plate surface heating unit of the class described with means to exert a constant pressure against the insulated foil heater to hold it firmly against the underside of the plate and prevent it from separating from the plate at high operating temperatures.
A further object of the present invention is to provide a solid plate surface heating unit of the class described with a thin, high-quality, insulating support pad to raise the thermal efficiency of the surface heating unit. In turn, the support pad is braced by a reinforced reflector pan that prevents the support pad from sagging at high operating temperatures.